Method for processing chromium oxide-containing substances in large quantities, method for utilizing the processed substances, and products comprising the processed substances

ABSTRACT

A method of rapidly reducing chromium oxide-containing slag and the like in large quantities, in a simplified manner and without requiring high temperatures. Chromium oxides are reduced with at least one of elementary sulfur and compounds of sulfur having a valence less than 6. For an aqueous solution of the sulfur component, desirably, its sulfur content is more than 0.03% by weight. As the sulfur source, preferred is blast furnace slag (e.g., non-aged, gradually-cooled blast furnace slag) that is discharged in large quantities in the iron industry. Cr 6+  in chromium oxides is reduced in one of the following ways: (a) Chromium oxide-containing substances are sprayed with or immersed in blast furnace slag-released water that has been used in cooling blast furnace slag. (b) Chromium oxide-containing substances are mixed with blast furnace slag, and then kept in an air atmosphere. Optionally, the mixture is sprayed with blast furnace slag-released water. (c) Chromium oxide-containing substances are mixed with blast furnace slag, and then steamed. (d) Prior to the reduction treatment, chromium oxide-containing substances are preferably steamed. 100 parts by weight of reduced chromium oxide-containing compounds are mixed with from 0.1 to 90 parts by weight of sulfur-containing slag, and used in various civil engineering works.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for processing chromiumoxide-containing substances, for example, stainless steel slag asdischarged in refining stainless steel, chromium slag as discharged inproducing chromium compounds such as sodium bichromate, slag of moltenwastes, sewage sludge, slag of molten sewage sludge and the like,thereby to prevent the release of Cr⁶⁺ from those substances.

BACKGROUND OF THE INVENTION

[0002] Stainless steel slag as discharged in refining stainless steel,and chromium slag as discharged in producing chromium compounds such assodium bichromate, contain chromium oxides at a level of a few %. A partof those chromium oxides is often oxidized to Cr⁶⁺, depending on theworking conditions, and Cr⁶⁺ is released from them.

[0003] Where stainless steel slag and chromium slag are used asmaterials in roadbeds, materials in temporary works, fillers in civilengineering reclamation works and the like, it is indispensable thatCr⁶⁺ is not released from the slag.

[0004] Recently, recycling of slag as formed by melting ashes fromgarbage incinerators, sludge and the like in roadbeds, tiles and thelike is now under consideration in the industry. However, the slagformed from some types of ashes from garbage incinerators, sludge andthe like will often release Cr⁶⁺ to make it difficult to recycle theslag.

[0005] Accordingly, for safely recycling such slag in roadbeds, fillersin civil engineering reclamation works and the like, there have beenproposed some new techniques of preventing the release of Cr⁶⁺ from theslag.

[0006] For example, for preventing the release of Cr⁶⁺ from stainlesssteel slag, there has been disclosed a method of adding aluminum ash andmagnesia-based industrial wastes to stainless steel slag (see JapanesePatent Application Laid-Open (JP-A) No. Hei-6-171993).

[0007] When industrially carrying out the disclosed method, the intendedaluminum ash and magnesia-based industrial wastes are previouslyintroduced into a slag-receiving kiln, into which molten stainless steelslag is poured, stirred and mixed by the energy of the falling slagmelt.

[0008] In this method, however, the slag melt is highly viscous, and asto this is different from molten steel, and it is difficult to produce ahomogeneously mixed condition by the stirring and mixing means assistedonly by the energy of the falling slag melt, resulting in the failure tocompletely stabilize all chromium oxides in the slag to thereby preventthe release of Cr⁶⁺ from the slag.

[0009] Another method was proposed, of adding a divalent S compound suchas FeS or the like to molten slag resulting from decarburizing andrefining stainless steel. The resulting slag is then reduced to recoverCr in the slag. The slag is then stirred with an inert gas streamintroduced thereinto, thereby making the slag have an S content of notless than 0.20% by weight so as not to release Cr⁶⁺ therefrom (see JP-AHei-8-104553).

[0010] In this method, an inert gas stream is introduced into the slagmelt to homogeneously mix the slag melt with the additive, therebystabilizing the chromium oxides existing in the slag. This method iseffective in preventing the release of Cr⁶⁺ from the slag, but requiresthe troublesome operation of introducing such an inert gas stream intothe reduced slag melt while using the additive for lowering theviscosity of the slag melt. Therefore, the method is not economical.

[0011] In addition, the method is further troublesome in that, if theadditive is added to the slag melt in the refining furnace in order toensure the homogeneous mixing of the slag melt with the additive, theadditive will contaminate the steel melt.

[0012] On the other hand, for preventing the release of Cr⁶⁺ fromchromium slag as discharged in producing chromium compounds, a methodhas been employed of reductionally roasting the slag to thereby reduceCr⁶⁺ to Cr³⁺ for rendering the slag harmless. Economically, however,this method is troublesome in that it requires a roasting step.

[0013] Sewage sludge is incinerated to reduce its volume prior to beingused for reclamation. To prevent the release of Cr⁶⁺ from theincinerated ashes, a method is used for controlling the air ratio duringincineration to be less than 1 (see Journal of the Drainage WorksAssociation of Japan, Vol. 38, No. 378, pp. 29-32, 1994).

[0014] However, as pointed out in this publication, it is extremelydifficult to ensure the optimum operation for a variety of sludgesderived from various sites and having different properties, and the sameapplies also to the incineration of industrial wastes.

[0015] To reduce Cr⁶⁺ in chromium oxide-containing substances at roomtemperature, there is generally employed a method using ferrous sulfateas the reducing agent. In this method, however, it is difficult toefficiently lower the Cr⁶⁺ content of the processed substances to anenvironmentally acceptable level.

[0016] Where chromium oxide-containing substances are reduced orincinerated according to the methods noted above, and the thus-processedsubstances are recycled in roadbeds, fillers in civil engineeringreclamation works, temporary works and the like, they are checked as towhether or not chromium oxides were fully reduced, prior to beingshipped. If the processed substances contain non-reduced chromiumoxides, they cannot be recycled in roadbeds, fillers in civilengineering reclamation works, temporary works and the like.

OBJECTS OF THE INVENTION

[0017] An object of the present invention is to solve the problems inthe prior art noted above, and to provide an industrial, simple andeconomical method for processing chromium oxide-containing substances,such as stainless steel slag, chromium slag, slag of molten wastes,sewage sludge, slag of molten sewage sludge, soil contaminated withchromium oxides and the like, thereby to completely prevent the releaseof Cr⁶⁺ from those chromium oxide-containing substances.

[0018] Another object of the invention is to provide a method forprocessing chromium oxide-containing substances such as those mentionedabove within a short period of time without significantly increasing thevolume of the substances processed to attain the intended results.

[0019] Still another object of the invention is to provide a method ofrecycling chromium oxide-containing substances, and to provide materialsfor roadbeds, fillers in civil engineering reclamation works, materialsfor temporary works, materials for civil engineering and constructionworks, etc.

SUMMARY OF THE INVENTION

[0020] Having carefully studied the problems in the prior art notedabove, the present inventors have found that, in the prior art in whichthe defined reduction of chromium is attained in a high-temperaturecondition for melting, roasting or incineration, the reduction conditionis difficult to control and the defined condition is not economical.

[0021] On the basis of this finding, we have tried methods of reducingCr⁶⁺ at room temperature or at temperatures near room temperature.Specifically, we have made various studies and experiments for reactingvarious chromium oxide-containing substances such as those mentionedabove with different aqueous solutions containing a variety of reducingagents, thereby to reduce Cr⁶⁺ in those substances.

[0022] As a result, we have found that, when water that contains sulfurhaving a valence less than 6, especially water that contains sulfur ionsand sulfur as released from the non-aged material of blast furnace slag,is used, then chromium existing in chromium oxide-containing substancescan be reduced with high reactivity in an industrial, simple andeconomical manner. On the basis of this finding, we have achieved theobjects of the present invention.

[0023] In addition, based on this finding, we have further studied andhave made the following findings [1] to [3]:

[0024] [1] Using any of sulfur-containing substances listed below as <1>to <4>, the release of Cr⁶⁺ from chromium oxide-containing substances isextremely effectively prevented.

[0025] <1> Water that has been used for cooling blast furnace slag(hereinafter referred to as blast furnace slag-released water).

[0026] <2> Slag containing sulfur and/or compounds of sulfur having avalence less than 6, for example, gradually-cooled blast furnace slagspontaneously aged for less than 3 months (hereinafter referred to asnon-aged, gradually-cooled blast furnace slag), slag discharged from thepre-treatment of molten pig iron, etc.

[0027] <3> Elemental sulfur, and substances containing elemental sulfur,such as sinter (flowers of sulfur) as recovered from sulfur-containinghot springs and the like (hereinafter referred to as elementalsulfur-containing substances).

[0028] <4> Compounds of sulfur having a valence of less than 6, such assodium thiosulfate, iron sulfide, hydrogen sulfide and the like(hereinafter referred to as compounds of sulfur with a valence of lessthan 6).

[0029] Generically, those substances <1> to <4> set forth above will behereinafter referred to as sulfur-containing substances.

[0030] [2] Using any of those sulfur-containing substances <1> to <4>noted above, the release of Cr⁶⁺ from chromium oxide-containingsubstances is extremely effectively prevented according to any of thefollowing processing methods (a) to (d).

[0031] (a) They are kept in an aerial atmosphere.

[0032] (b) Water is sprayed over them.

[0033] (c) Chromium oxide-containing substances are immersed insulfur-containing substances.

[0034] (d) Steam is applied to them.

[0035] The above mentioned item (a) is a further surprising new finding.The release Cr⁶⁺ from chromium oxide-containing substances can beeffectively prevented without using such aqueous solutions containingreduction agents as used in the first experiment described on page 6,2nd and 3rd paragraph in this specification, according to the processingmethod of mixing the chromium oxide-containing substances with thesulfur-containing slag noted in [1]<2> and then simply leaving theresulting mixture in an air atmosphere.

[0036] [3] Using the sulfur-containing substances <1> to <4> optionallycombined with any divalent iron-containing substances (hereinafterreferred to as Fe(II)-containing substances), such as ferrous sulfate,produces the following advantages (1) and (2).

[0037] (1) Even for blast furnace slag-released water having a lowconcentration of reducing sulfur, of which the concentration may vary inthe water, any of the elemental sulfur-containing substances, thecompounds of sulfur with a valence less than 6 or the Fe(II)-containingsubstances may be added thereto or sprayed thereover, and chromiumoxide-containing substances to be processed are immersed in theresulting blast furnace slag-released water, whereby Cr⁶⁺ in thechromium oxide-containing substances can be reduced within a shortperiod of time, resulting in that the release of Cr⁶⁺ from thethus-processed chromium oxide-containing substances is completelyprevented.

[0038] (2) Even for slag that releases a large amount of Cr⁶⁺ accordingto the metal release test as set forth in Notification No. 46 of theEnvironment Agency of Japan, or for slag having a low degree ofporosity, the chromium oxide-containing substances in the slag of thattype may be mixed with any of the elemental sulfur-containingsubstances, and steam may be applied thereto, whereby the slag can beprocessed within a short period of time without increasing the volume ofthe processed slag, resulting in that the release of Cr⁶⁺ from thechromium oxide-containing substances in the thus-processed slag iscompletely prevented.

[0039] As preferred examples of the sulfur-containing substances havingan overall sulfur content of not less than 10% by weight for sulfurand/or compounds of sulfur having a valence less than 6, there can bementioned elemental sulfur; elemental sulfur-containing substances suchas sinter recovered from sulfur-containing hot springs, etc.; andcompounds of sulfur with a valence less than 6 such as sodiumthiosulfate, iron sulfide, hydrogen sulfide, etc. These may combined foruse in the invention.

[0040] Where blast furnace slag-released water is sprayed over chromiumoxide-containing substances to be processed therewith, the thus-sprayedsubstances are preferably kept in an aerial atmosphere to complete thereduction processing.

[0041] Also preferably, steam is applied to the chromiumoxide-containing substances to be processed according to the inventionprior to the intended reduction processing of the substances.

[0042] From 0.1 to 90 parts by weight of sulfur-containing slag may beadded to and mixed with 100 parts by weight of the reduced, chromiumoxide-containing substances to give materials for roadbeds, fillers incivil engineering reclamation works, materials for temporary works, andmaterials for civil engineering and construction works.

BRIEF DESCRIPTION OF THE DRAWING

[0043]FIG. 1 is a graph showing the relationship between the time forsteaming stainless steel slag and the amount of Cr⁶⁺ released from theslag.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0044] The invention will be described in detail hereinunder withreference to its preferred embodiments.

[0045] The first aspect of the invention is a method of reducingchromium oxide-containing substances, such as chromium oxide-containingslag, sewage sludge, contaminated soil and the like, in large quantitiesand rapidly. The method of the invention requires neither particularlyhigh temperatures nor specific large-scale equipment.

[0046] According to the method of the invention, Cr⁶⁺ in chromiumoxide-containing substances is reduced by a reducing sulfur (at leastone of elemental sulfur and sulfur ions having a valence of less than 6,e.g. 2 or 4) at atmospheric temperature or at a temperature of up toabout 200° C. when vapor is optionally used. As the method does not useany slag melt, it does not require particularly high temperatures. Inother words, according to the method of the invention, chromiumoxide-containing substances can be reduced efficiently at lowtemperatures.

[0047] In one preferred embodiment of the method of the invention,chromium oxide-containing substances are kept in contact with an aqueoussolution having an overall sulfur content greater than 0.03% by weight(0.3 g/liter) for sulfur and/or compounds of sulfur having a valence ofless than 6 therein.

[0048] The present inventors carried out an experiment which is asfollows: A reagent of CrO₃ was dissolved in pure water to prepare anaqueous solution having a Cr⁶⁺ content of 100 mg/liter. To 10 liters ofthe thus-prepared aqueous solution, was added 100 g of non-aged,gradually-cooled blast furnace slag, which had been left for 1 weekafter having been cooled and ground to have a predetermined grain size,or of the gradually-cooled blast furnace slag that had beenspontaneously aged for 6 months, and was shaken for 6 hours, whereuponthe Cr⁶⁺ content of the thus-shaken aqueous solution was measured todetermine the decrease (mg) in the Cr⁶⁺ content of the solution.

[0049] The data are shown in Table 1.

[0050] In Table 1, the reducibility indicates the decrease (mg) in Cr⁶⁺per kg of the gradually-cooled blast furnace slag used.

[0051] It is known that the non-aged, gradually-cooled blast furnaceslag, depending on its grain size, has a degree of reducibility about 10to 100 times that of the gradually-cooled blast furnace slag asspontaneously aged for 6 months.

[0052] It is also known that slag having a smaller grain size has ahigher degree of reducibility.

[0053] The reason why the gradually-cooled blast furnace slag has theability to reduce Cr⁶⁺ is because the sulfur component existing in theslag dissolves in water to reduce Cr⁶⁺.

[0054] The present inventors carried out another experiment which is asfollows: A reagent of CrO₃ was dissolved in pure water to prepare anaqueous solution having a Cr⁶⁺ content of 100 mg/liter. To 10 liters ofthe thus-prepared aqueous solution, was added 100 g of blast furnaceslag-released water containing reducing sulfur having a valence of lessthan 6 in an amount of 0.5% by weight, and was shaken for 6 hours,whereupon the Cr⁶⁺ content of the thus-shaken aqueous solution wasmeasured to determine the decrease (mg) in the Cr⁶⁺ content of thesolution.

[0055] The data are shown in Table 2.

[0056] In Table 2, the reducibility indicates the decrease (mg) in Cr⁶⁺per kg of the blast furnace slag-released water used.

[0057] Table 2 shows that the blast furnace slag-released water also hasthe ability to reduce Cr⁶⁺, which is nearly comparable to that of thenon-aged, gradually-cooled blast furnace slag in terms of the value perthe unit weight.

[0058] In view of this, we clarified the mechanism of action of thosereducing substances. Table 3 shows the concentrations of sulfurs ofdifferent types in the gradually-cooled blast furnace slag as used forreducing Cr⁶⁺, before and after the reducing treatment using it.

[0059] Comparing the sulfur concentrations before and after the reducingtreatment, it is seen that the concentrations of sulfurs except S²⁻ werelowered after the reducing treatment.

[0060] In addition, we measured the concentrations of sulfurs ofdifferent types in the reduced aqueous solutions, and found that mostsulfurs therein were in the form of SO₄ ²⁻.

[0061] Accordingly, it is believed that S⁰, S₂O₃ ²⁻ and SO₄ ²⁻ in thegradually-cooled blast furnace slag dissolve in water, and that ofthese, S⁰ and S₂O₃ ²⁻ are oxidized to SO₄ ²⁻ while reducing Cr⁶⁺.

[0062] The reason why the ability to reduce Cr⁶⁺ in the slag powderhaving a smaller grain size is higher is because the slag powder havinga smaller grain size has a larger specific surface area so that the S⁰and S₂O₃ ²⁻ therein are easier to dissolve in water.

[0063] The first aspect of the present invention has been completed onthe basis of the findings noted above.

[0064] According to the invention, when an aqueous solution containingsulfur and/or compounds of sulfur having a valence less than 6 iscontacted with chromium oxide-containing substances, it reduces chromiumto stable chromium compounds of Cr(OH)₃ and the like, whereby therelease of chromium oxides from the chromium oxide-containing substancesis prevented.

[0065] Even if some chromium ions are released from chromiumoxide-containing substances and dissolve to the aqueous solution, theyare reduced in the aqueous solution so that the release of Cr⁶⁺ from thechromium oxide-containing substances is prevented.

[0066] In the preferred embodiment of the first aspect of the invention,the overall sulfur content of the aqueous solution that contains sulfurand/or compounds of sulfur having a valence less than 6 is desirablylarger than 0.03% by weight.

[0067] This is because an aqueous solution having an overall sulfurcontent of not larger than 0.03% by weight for sulfur and/or compoundsof sulfur having a valance less than 6 therein takes a few months orlonger to reduce chromium oxide-containing substances, and is thereforedifficult to use in industrial processes.

[0068] As sulfur and/or compounds of sulfur having a valence less than6, and their sources, which are used in the first aspect of theinvention, there are preferred one or more sulfur-containing substancesthat may be selected from the following items <1> to <4>, which,however, are not limitative. Apart from those mentioned below, any othersubstances containing sulfur and/or compounds of sulfur having a valenceless than 6 are employable herein. In other words, the sources ofreducing sulfurs to be used in the invention are not specificallydefined, but, for example, may be blast furnace slag that is dischargedin large quantities in the iron and steel industry.

[0069] <1> Water as used for cooling blast furnace slag (blast furnaceslag-released water).

[0070] <2> Slag containing sulfur and/or compounds of sulfur having avalence less than 6, for example, gradually-cooled blast furnace slag asspontaneously aged for less than 3 months (non-aged, gradually-cooledblast furnace slag), slag as discharged in the pre-treatment of pig ironmelt, etc.

[0071] <3> Elemental sulfur, and substances containing elemental sulfur,such as sinter as recovered from sulfur-containing hot springs and thelike (elemental sulfur-containing substances).

[0072] <4> Compounds of sulfur having a valence less than 6, such assodium thiosulfate, iron sulfide, hydrogen sulfide and the like(compounds of sulfur with a valence less than 6). TABLE 1 ReducibilityType of Slag Grain Size (mm) (mg-Cr⁶⁺/kg-slag) Non-aged, 13.2.-4.75 1220 Gradually-cooled  4.75-0.425 1460 Blast Furnace 0.425-0.075 1720Slag(*) <0.075 2040 Gradually-cooled 132.-4.75 15 Blast Furnace 4.75-0.425 25 Slag as 0.425-0.075 85 spontaneously <0.075 263 aged for6 months

[0073] TABLE 2 Reducibility Type of (mg-Cr⁶⁺/kg-blast furnaceSlag-released Water Grain Size (mm) slag-released slag) Blast FurnaceSlag- — 1340 released Water(*)

[0074] TABLE 3 Before Reducing Treatment After Reducing Treatment (wt.%) (wt. %) S as S²⁻ 0.04 0.04 S as S⁰ 0.22 <0.005 S as S₂O₃ ²⁻ 0.31 0.02S as SO₄ ²⁻ 0.52 0.19

[0075] Based on the first aspect of the invention noted above, thepresent inventors have further studied and obtained the followingfindings:

[0076] [1] Using any of sulfur-containing substances of the followingitems <1> to <4>, the release of Cr⁶⁺ from chromium oxide-containingsubstances is extremely effectively prevented.

[0077] <1> Water as used for cooling blast furnace slag (blast furnaceslag-released water).

[0078] <2> Slag containing sulfur and/or compounds of sulfur having avalence less than 6, for example, gradually-cooled blast furnace slag asspontaneously aged for less than 3 months (non-aged, gradually-cooledblast furnace slag), slag as discharged in the pre-treatment of pig ironmelt, etc.

[0079] <3> Elemental sulfur, and sinter as recovered fromsulfur-containing hot springs (elemental sulfur-containing substances).

[0080] <4> Compounds of sulfur having a valence less than 6, such assodium thiosulfate, iron sulfide, hydrogen sulfide and the like(compounds of sulfur with a valence less than 6).

[0081] [2] Using any of the sulfur-containing substances <1> to <4>noted above, the release of Cr⁶⁺ from chromium oxide-containingsubstances is extremely effectively prevented according to any of thefollowing processing methods (1) to (4).

[0082] (1) A Method of Keeping Them in an Aerial Atmosphere.

[0083] Chromium oxide-containing substances are mixed with slag thatcontains sulfur and/or compounds of sulfur having a valence less than 6,and the resulting mixture is left in an aerial atmosphere.

[0084] (2) A Method of Spraying Water Over Them.

[0085] (a) Blast furnace slag-released water is sprayed over chromiumoxide-containing substances.

[0086] (b) Chromium oxide-containing substances are mixed with slag thatcontains sulfur and/or compounds of sulfur having a valence less than 6,and water and/or blast furnace slag-released water is sprayed over theresulting mixture.

[0087] (3) A Method of Immersing Chromium Oxide-containing Substances inSulfur-containing Substances.

[0088] (a) Chromium oxide-containing substances are immersed in blastfurnace slag-released water.

[0089] (b) Chromium oxide-containing substances are immersed in blastfurnace slag-released water to which are added substances that containsulfur and/or compounds of sulfur having a valence less than 6.

[0090] (4) A Method of Applying Steam to Them.

[0091] (a) Chromium oxide-containing substances are mixed withsubstances that contain sulfur and/or compounds of sulfur having avalence less than 6, and steam is applied to the resulting mixture.

[0092] (b) Chromium oxide-containing substances are mixed with slag thatcontains sulfur and/or compounds of sulfur having a valence less than 6,and steam is applied to the resulting mixture.

[0093] (c) Chromium oxide-containing substances are mixed with slag thatcontains sulfur and/or compounds of sulfur having a valence less than 6along with substances that contain sulfur and/or compounds of sulfurhaving a valence less than 6, and steam is applied to the resultingmixture.

[0094] [3] Using the sulfur-containing substances <1> to <4> optionallycombined with any of divalent iron-containing substances(Fe(II)-containing substances), such as ferrous sulfate, produces thefollowing advantages (1) and (2).

[0095] (1) Even for the blast furnace slag-released water <1> having asmall concentration of reducing sulfur, of which the concentration mayvary in the water <1>, substances that contain sulfur and/or compoundsof sulfur having a valence less than 6 may be added thereto, andchromium oxide-containing substances to be processed are immersed in theresulting blast furnace slag-released water, whereby Cr⁶⁺ in thechromium oxide-containing substances can be reduced within a shortperiod of time, resulting in that the release of Cr⁶⁺ from thethus-processed chromium oxide-containing substances is completelyprevented.

[0096] (2) Even for slag that releases a large amount of Cr⁶⁺ in themetal release test set forth in the Notification No. 46 of theEnvironment Agency of Japan, or for slag having a low degree ofporosity, the chromium oxide-containing substances in the slag of thattype, or both the chromium oxide-containing substances therein, and slagthat contains sulfur and/or compounds of sulfur having a valence lessthan 6 may be mixed with substances that contain sulfur and/or compoundsof sulfur having a valence less than 6, and steam may be appliedthereto, whereby the slag that contains chromium oxide-containingsubstances can be processed within a short period of time withoutincreasing the volume of the processed slag, resulting in that therelease of Cr⁶⁺ from the chromium oxide-containing substances in thethus-processed slag is completely prevented.

[0097] Accordingly, in another preferred embodiment of the first aspectof the invention, chromium oxide-containing substances are mixed withslag containing sulfur and/or compounds of sulfur having a valence lessthan 6, and the resulting mixture is kept in an aerial atmosphere.

[0098] In this embodiment, non-aged, gradually-cooled blast furnace slagand/or slag discharged from pre-treatment of molten pig iron arepreferred as the slag containing sulfur and/or compounds of sulfurhaving a valence less than 6, and more preferred is non-aged,gradually-cooled blast furnace slag.

[0099] In still another preferred embodiment of the first aspect of theinvention, blast furnace slag-released water, which is water as used forcooling blast furnace slag, is sprayed over chromium oxide-containingsubstances.

[0100] In still another preferred embodiment thereof, chromiumoxide-containing substances are mixed with slag containing sulfur and/orcompounds of sulfur having a valence less than 6, and water and/or blastfurnace slag-released water, which is water as used for cooling blastfurnace slag, is sprayed over the resulting mixture.

[0101] In this embodiment, non-aged, gradually-cooled blast furnace slagand/or slag as discharged from the pre-treatment of molten pig iron arepreferred as the slag containing sulfur and/or compounds of sulfurhaving a valence less than 6, and more preferred is non-aged,gradually-cooled blast furnace slag.

[0102] In still another preferred embodiment of the first aspect of theinvention, chromium oxide-containing substances are immersed in blastfurnace slag-released water, which is water that has been used forcooling blast furnace slag.

[0103] In still another preferred embodiment thereof, chromiumoxide-containing substances are immersed in blast furnace slag-releasedwater to which are added substances containing sulfur and/or compoundsof sulfur having a valence less than 6.

[0104] In those embodiments, preferred are sulfur-containing substanceshaving an overall sulfur content of not less than 10% by weight forsulfur and/or compounds of sulfur having a valence less than 6 therein,as the substances containing sulfur and/or compounds of sulfur having avalence less than 6.

[0105] The sulfur-containing substances having an overall sulfur contentof not less than 10% by weight for sulfur and/or compounds of sulfurhaving a valence less than 6 therein for use in those embodiments arenot specifically defined, but preferred are, for example, elementalsulfur; substances containing elemental sulfur such as sinter asrecovered from sulfur-containing hot springs (elementalsulfur-containing substances); and compounds of sulfur having a valenceless than 6, such as sodium thiosulfate, iron sulfide, hydrogen sulfideand the like (compounds of sulfur with a valence less than 6). Two ormore of these may be used in combination.

[0106] As the substances containing compounds of sulfur having a valenceless than 6, preferred are those that hardly react with oxygen dissolvedin water.

[0107] This is because, if sulfur-containing substances that easilyreact with oxygen dissolved in water are used, they will react withoxygen in water prior to acting on chromium oxide-containing substances,thereby losing their ability to completely reduce Cr⁶⁺ in the chromiumoxide-containing substances.

[0108] The reducing sulfur concentration in blast furnace slag-releasedwater varies, depending on the season and the weather, and may be oftennot more than 0.03% by weight. If blast furnace slag-released waterhaving such a low reducing sulfur concentration is used, it takes a longtime to reduce Cr⁶⁺ in chromium oxide-containing substances.

[0109] In view of this, the present inventors have further studied andmade various experiments in order to find an effective method capable ofprocessing chromium oxide-containing substances within a short period oftime even with blast furnace slag-released water having a low reducingsulfur concentration of not more than 0.03% by weight, therebycompletely to prevent the release of Cr⁶⁺ from the thus-processedchromium oxide-containing substances.

[0110] As a result, we have found that, when substances containingsulfur and/or compounds of sulfur having a valence less than 6 are addedto blast furnace slag-released water and chromium oxide-containingsubstances are immersed in the resulting mixture, then the release ofCr⁶⁺ from the thus-processed chromium oxide-containing substances can becompletely prevented, even though the blast furnace slag-released waterused has a low reducing sulfur concentration of not more than 0.03% byweight and even though the processing is effected within a short periodof time.

[0111] Accordingly, even for blast furnace slag-released water having alow reducing sulfur concentration of not more than 0.03% by weight, ifsubstances containing sulfur and/or compounds of sulfur having a valenceless than 6 are added thereto, chromium oxide-containing substances maybe immersed in the resulting mixture only within a short period of timethereby completely to prevent the release of Cr⁶⁺ from thethus-processed chromium oxide-containing substances.

[0112] The reducing sulfur concentration in blast furnace slag-releasedwater as referred to herein is obtained by subtracting the sulfurcontent of SO₄ ²⁻ from the overall sulfur concentration in the water.

[0113] According to the preferred embodiment noted above, the substancescontaining sulfur and/or compounds of sulfur having a valence less than6 added to blast furnace slag-released water shall control the reducingsulfur concentration in the thus-mixed water, with which, therefore,Cr⁶⁺ in the chromium oxide-containing substances processed is rapidlyreduced.

[0114] In this preferred embodiment, the amount of the substancescontaining sulfur and/or compounds of sulfur having a valence less than6 to be added to blast furnace slag-released water is preferably socontrolled that the reducing sulfur concentration in the blast furnaceslag-released water containing those substances is greater than 0.03% byweight.

[0115] This is because, if the reducing sulfur concentration in theblast furnace slag-released water used is not more than 0.03% by weight,it is difficult to completely reduce Cr⁶⁺ in chromium oxide-containingsubstances with the water within a short period of time.

[0116] Also preferably, divalent iron-containing substances such asferrous sulfate, ferrous chloride and the like (Fe(II)-containingsubstances) may be added to blast furnace slag-released water for use inthe invention.

[0117] Adding divalent iron ions to blast furnace slag-released waterensures more rapid and more complete reduction of Cr⁶⁺ in chromiumoxide-containing substances as processed with the divalentiron-containing water.

[0118] In still another preferred embodiment of the first aspect of theinvention, chromium oxide-containing substances are mixed withsubstances containing sulfur and/or compounds of sulfur having a valenceless than 6, and steam is applied to the resulting mixture.

[0119] The present inventors have further studied and made variousexperiments in order to find an industrial, simple, economical andeffective method capable of processing slag that releases Cr⁶⁺ in anamount of 10 mg/liter or more in the metal release test set forth in theNotification No. 46 of the Environment Agency of Japan, or slag having adegree of porosity of about 5% or lower, within a short period of timewithout increasing the volume of the processed slag, thereby completelyto prevent the release of Cr⁶⁺ from the thus-processed chromiumoxide-containing substances in the slag.

[0120] As a result, we have found that, when the slag of that typecontaining therein chromium oxide-containing substances is mixed withsubstances containing sulfur and/or compounds of sulfur having a valenceless than 6, and steam is applied to the resulting mixture, then theslag can be processed within a short period of time without increasingthe volume of the processed slag, resulting in that the release of Cr⁶⁺from the chromium oxide-containing substances in the thus-processed slagis completely prevented.

[0121] In this embodiment, preferred are sulfur-containing substanceshaving an overall sulfur content of not less than 10% by weight forsulfur and/or compounds of sulfur having a valence less than 6 therein,as the substances containing sulfur and/or compounds of sulfur having avalence less than 6.

[0122] This is because the amount to be added of sulfur-containingsubstances having a larger overall sulfur content of sulfur and/orcompounds of sulfur having a valence less than 6, relative to thechromium oxide-containing substances existing in the slag to beprocessed, may be smaller, resulting in that the increase in the volumeof the processed slag may be minimized, that the amount of the steam tobe used may be decreased, and that the processed slag is easy totransport.

[0123] The sulfur-containing substances having an overall sulfur contentof not less than 10% by weight of sulfur and/or compounds of sulfurhaving a valence less than 6 therein for use in this embodiment are notspecifically defined, but preferred are, for example, elemental sulfur;substances containing elemental sulfur such as sinter as recovered fromsulfur-containing hot springs (elemental sulfur-containing substances);and compounds of sulfur having a valence less than 6, such as sodiumthiosulfate, iron sulfide and the like (compounds of sulfur with avalence less than 6). Two or more of these may be used in combination.

[0124] In still another preferred embodiment of the invention, chromiumoxide-containing substances are mixed with slag containing sulfur and/orcompounds of sulfur having a valence less than 6, and steam is appliedto the resulting mixture.

[0125] In this embodiment, preferred are non-aged, gradually-cooledblast furnace slag and/or slag discharged from the pre-treatment ofmolten pig iron, as the slag containing sulfur and/or compounds ofsulfur having a valence less than 6; and more preferred is non-aged,gradually-cooled blast furnace slag.

[0126] In still another preferred embodiment of the invention, chromiumoxide-containing substances are mixed with slag containing sulfur and/orcompounds of sulfur having a valence less than 6 and also withsulfur-containing substances containing sulfur and/or compounds ofsulfur having a valence less than 6, and steam is applied to theresulting mixture.

[0127] In this embodiment, preferred are non-aged, gradually-cooledblast furnace slag and/or slag as discharged from the pre-treatment ofmolten pig iron melt, as the slag containing sulfur and/or compounds ofsulfur having a valence less than 6; and more preferred is non-aged,gradually-cooled blast furnace slag.

[0128] In this, also preferred are sulfur-containing substances havingan overall sulfur content of not less than 10% by weight for sulfurand/or compounds of sulfur having a valence less than 6, as thesubstances containing sulfur and/or compounds of sulfur having a valenceless than 6.

[0129] This is because the amount to be added of sulfur-containingsubstances having a larger overall sulfur content for sulfur and/orcompounds of sulfur having a valence less than 6 may be smaller,resulting in that the increase in the volume of the processed chromiumoxide-containing substances may be minimized, that the amount of thesteam to be used may be decreased, and that the processed chromiumoxide-containing substances are easy to transport.

[0130] The sulfur-containing substances having an overall sulfur contentof not less than 10% by weight for sulfur and/or compounds of sulfurhaving a valence less than 6 therein for use in this embodiment are notspecifically defined, but preferred are, for example, elemental sulfur;substances containing elemental sulfur such as sinter as recovered fromsulfur-containing hot springs (elemental sulfur-containing substances);and compounds of sulfur having a valence less than 6, such as sodiumthiosulfate, iron sulfide and the like (compounds of sulfur with avalence less than 6). Two or more of these may be used in combination.

[0131] In this embodiment, the steam applied acts on chromiumoxide-containing substances to thereby make Cr⁶⁺ in those substanceseasily releasable, or that is, make it easily reducible. In this, inaddition, sulfur and compounds of sulfur having a valence less than 6are released from the slag that contains sulfur and/or compounds ofsulfur having a valence less than 6, such as gradually-cooled blastfurnace slag, and from the sulfur-containing substances that containsulfur and/or compounds of sulfur having a valence less than 6, andthose sulfur and sulfur compounds are oxidized while reducing Cr⁶⁺ inthe chromium oxide-containing substances to Cr³⁺.

[0132] Therefore, according to this embodiment in which are combinedslag that contains sulfur and/or compounds of sulfur having a valenceless than 6, and sulfur-containing substances that contain sulfur and/orcompounds of sulfur having a valence less than 6, chromiumoxide-containing compounds can be processed within a much shorter periodof time even when the amount of the slag added is reduced.

[0133] The reducing mechanism in this embodiment is as follows:

[0134] When steam is applied to chromium oxide-containing substanceshaving a porosity of 5% or smaller and a grain size of 40 mm or smaller,it takes about 1 day to make 50% of Cr⁶⁺ in the substances easilyreleasable, about 2 days to make 80% of Cr⁶⁺ in the substances easilyreleasable, and about 3 days to make 100% of Cr⁶⁺ in the substanceseasily releasable.

[0135] On the other hand, when steam is applied to non-aged,gradually-cooled blast furnace slag, the amount dissolving in water ofsulfur with a valence less than 6 in the slag is proportional to theperiod of time of the steam application, and it takes about 10 daysbefore the entire amount of the sulfur of that type dissolves in water.

[0136] On the other hand, the elemental sulfur existing in the slagdissolves in water within about 3 days.

[0137] Accordingly, where only non-aged, gradually-cooled blast furnaceslag is added to chromium oxide-containing substances, 100% of Cr⁶⁺ inthe substances is made easily releasable, or that is, easily reduciblewithin about 3 days, whilst the releasing speed of sulfur with a valenceless than 6 from the slag is low and the sulfur of that type takes 3days or longer to be released from the slag.

[0138] Needlesstosay, if an excess amount of non-aged, gradually-cooledblast furnace slag is added, the processing of chromium oxide-containingsubstances may be completed within 3 days. However, adding such anexcess amount of slag is troublesome in that the volume of the processedsubstances increases, that the amount of the steam to be appliedincreases, and that the processed substances are not easy to transport.

[0139] On the other hand, if, for example, elemental sulfur only isadded as the sulfur-containing substance, it dissolves in water withinabout 3 days, and its ability to reduce Cr⁶⁺ lasts for about 3 days.

[0140] On the other hand, it takes about 3 days to make 100% of Cr⁶⁺ inchromium oxide-containing substances having a porosity of 5% or smaller,to which is applied steam, easily releasable, or that is, easilyreducible. Therefore, if elemental sulfur only is added to thesubstances, its ability to reduce Cr⁶⁺ in the substances is lost beforeit acts on the substances to completely reduce Cr⁶⁺ therein.

[0141] Taking these facts into consideration, the present inventorstried the combination of non-aged, gradually-cooled blast furnace slagand substances that contains sulfur and/or compounds of sulfur having avalence less than 6, to which was applied steam, resulting in that Cr⁶⁺in the chromium oxide-containing substances was completely reducedwithin a short period of time without significantly increasing thevolume of the processed substances.

[0142] Specifically, in this embodiment, Cr⁶⁺ in chromiumoxide-containing substances is reduced essentially by the substancesthat contain sulfur and/or compounds of sulfur having a valence lessthan 6, such as elemental sulfur, sinter and the like, in the initialstage of 1 to 2 days while steam is applied to the system, and isreduced essentially by the slag that contains sulfur and/or compounds ofsulfur having a valence less than 6, such as non-aged, gradually-cooledblast furnace slag and the like, in the final stage of 2 to 3 days whilesteam is applied thereto.

[0143] There follow (I) preferred examples of the slag that containssulfur and/or compounds of sulfur having a valence less than 6, andthose of the blast furnace slag-released water, (II) preferred ranges ofthe amount to be added of the slag that contains sulfur and/or compoundsof sulfur having a valence less than 6, the amount of the blast furnaceslag-released water to be sprayed, and the reducing sulfur concentrationin the blast furnace slag-released water, and (III) preferred modes ofprocessing chromium oxide-containing substances, which are for theembodiments of the first aspect of the invention mentioned hereinabove.

[0144] (I) Preferred examples of slag that contains sulfur and/orcompounds of sulfur having a valence less than 6, and those f blastfurnace slag-released water:

[0145] As the slag that contains sulfur and/or compounds of sulfurhaving a valence less than 6, which is used in the invention, preferredare non-aged, gradually-cooled blast furnace slag and/or slag asdischarged in the pre-treatment of molten pig iron, and more preferredis non-aged, gradually-cooled blast furnace slag.

[0146] Non-aged, gradually-cooled Blast Furnace Slag:

[0147] The non-aged, gradually-cooled blast furnace slag for use in theinvention is slag that gives yellowish cloudy water, or that is, slagthat colors in the color identification test defined in JIS A5015,Appendix 1. Generally, this is gradually-cooled blast furnace slag asspontaneously aged for less than 3 months.

[0148] More preferred is non-aged, gradually-cooled blast furnace slagjust having been cooled and ground, or that is, within 1 week after thegrinding.

[0149] The grain size of the non-aged, gradually-cooled blast furnaceslag is not specifically defined, but a fine powdery one having asmaller grain size is better. This is because such fine powdery slaghaving a smaller grain size has a higher ability to reduce chromiumoxides.

[0150] More preferred is non-aged, gradually-cooled blast furnace slag,of which the grain size is so defined that the slag is, after havingbeen used for reducing chromium oxides, still effectively usable inindustrial products.

[0151] In this connection, for example, where stainless steel slag isreduced and is recycled as a material in roadbeds, if fine-powdery,non-aged, gradually-cooled blast furnace slag is used too much as thereducing agent for the stainless steel slag, the reduced slag would nolonger satisfy the requirement of the grain size distribution forroadbeds as stipulated in JIS A5015.

[0152] Accordingly, in this case, it is desirable to use non-aged,gradually-cooled blast furnace slag of which the grain size satisfiesthe requirement of the grain size distribution for roadbeds.

[0153] Slag Discharged in the Pre-treatment of Molten Pig Iron:

[0154] The slag as discharged in the pre-treatment of molten pig ironfor use in the invention is one to be discharged in the pre-treatmentstep of desulfurizing and dephosphorizing molten pig iron before themolten pig iron is transferred into a converter.

[0155] Blast Furnace Slag-released Water:

[0156] The blast furnace slag-released water for use in the invention ispreferably one that has been sprayed over non-aged, gradually-cooledblast furnace slag, or one that has been sprayed over hot slag justhaving been discharged from a blast furnace.

[0157] Specifically, as the blast furnace slag-released water for use inthe invention, preferred is one that has been sprayed overgradually-cooled blast furnace slag that has been spontaneously agedgenerally for less than 3 months, or one that has been sprayed over hotslag that has just been discharged from a blast furnace.

[0158] Where the blast furnace slag-released water of that type is used,the overall sulfur content of the water, which results from sulfurand/or compounds of sulfur having a valence less than 6, is preferablygreater than 0.03% by weight.

[0159] This is because, as mentioned hereinabove, blast furnaceslag-released water having an overall sulfur content of not ore than0.03% by weight for sulfur and/or compounds of sulfur having a valenceless than 6 takes a few months or longer to reduce chromiumoxide-containing substances, and is therefore difficult to use inindustrial processes.

[0160] (II) Preferred ranges of the amount to be added of slag thatcontains sulfur and/or compounds of sulfur having a valence less than 6,the amount of blast furnace slag-released water to be sprayed, and thereducing sulfur concentration in blast furnace slag-released water, areas follows:

[0161] The amount to be added, of the slag that contains sulfur and/orcompounds of sulfur having a valence less than 6, the amount of theblast furnace slag-released water to be sprayed, and the reducing sulfurconcentration in the blast furnace slag-released water are notspecifically defined, but are preferably as follows:

[0162] [1] In the embodiment of keeping the reduction system in anaerial atmosphere, in the embodiment of spraying water over thereduction system, and in the embodiment of introducing steam into thereduction system:

[0163] In those embodiments, where the typical blast furnace slag as inTable 3 is used as the slag that contains sulfur and/or compounds ofsulfur having a valence less than 6, such as non-aged, gradually-cooledblast furnace slag or the like, it is desirable that the amount of theslag to be added to the reduction system (hereinafter referred to asslag amount), the amount of the blast furnace slag-released water to besprayed over the system, and the amount to be added thereto of thesubstances that contain sulfur and/or compounds of sulfur having avalence less than 6 (e.g., elemental sulfur-containing substances,sulfur-containing substances with a valence less than 6) satisfy thefollowing requirements (1) and (2):

10>A>0.1  (1)

[0164] wherein;

[0165] A=[amount of slag added (wt.pts.)]×10+[amount of blast furnaceslag-released water sprayed (wt.pts.)]+[amount added of substancescontaining sulfur and/or compounds of sulfur having a valence less than6 (wt.pts.)]×500  (2)

[0166] In equation (2), parts by weight (wt.pts.) are all relative toone part by weight of the chromium oxide-containing substances beingprocessed, or to one mg/liter of Cr⁶⁺ having been released from theprocessed chromium oxide-containing substances, and all the amounts in(2) are proportional to the amount of the chromium oxide-containingsubstances being processed and that of Cr⁶⁺ having been released fromthe processed chromium oxide-containing substances.

[0167] If A in (1) is less than 0.1, outside the defined range, therelease of Cr⁶⁺ from the chromium oxide-containing substances processedis not inhibited sufficiently. On the other hand, even if A is more than10, also outside the defined range, the saturated effect of the reducingagents used for preventing the release of Cr⁶⁺ from the processedsubstances is not enhanced, and the use of such large amounts of thereducing agents is merely wasteful and hence uneconomical.

[0168] The amount of water to be sprayed over the mixture comprisingchromium oxide-containing substances and non-aged, gradually-cooledblast furnace slag is not specifically defined, but is preferably suchthat the chromium oxide-containing substances in the mixture may besaturated with the water that has been sprayed thereover.

[0169] This is because, when chromium oxides are reduced with non-aged,gradually-cooled blast furnace slag or the like, the sulfur and thecompounds of sulfur having a valence less than 6, which have beenreleased from the slag or the like to dissolve in water, act to reduceCr⁶⁺ existing in the chromium oxide-containing substances or Cr⁶⁺ thathas been released from the substances to dissolve in water.

[0170] [2] In the Embodiment of Immersing Chromium Oxide-containingSubstances in Reducing Baths:

[0171] When chromium oxide-containing substances are immersed blastfurnace slag-released water, the reducing sulfur concentration in thewater is preferably greater than 0.03% by weight.

[0172] This is because, if chromium oxide-containing substances areimmersed in blast furnace slag-released water having a reducing sulfurconcentration of not more than 0.03% by weight, Cr⁶⁺ in the substancesis difficult to completely reduce within a short period of time.

[0173] The reducing sulfur concentration in blast furnace slag-releasedwater as referred to herein is obtained by subtracting the sulfurcontent of SO₄ ²⁻ from the overall sulfur concentration in the water.

[0174] (III) Preferred Modes of Processing Chromium Oxide-containingSubstances:

[0175] In the embodiment of the invention mentioned hereinabove,chromium oxide-containing substances are mixed with slag that containssulfur and/or compounds of sulfur having a valence less than 6, such asnon-aged, gradually-cooled blast furnace slag or the like, and then keptin an air atmosphere.

[0176] Also in the other embodiments of the invention where chromiumoxide-containing substances are reduced by themselves or after havingbeen mixed with slag that contains sulfur and/or compounds of sulfurhaving a valence less than 6 optionally along with substances thatcontain sulfur and/or compounds of sulfur having a valence less than 6,the reduction system is preferably processed in the intended mannerwhile kept in an air atmosphere.

[0177] This is because the method of keeping the reduction system in anair atmosphere is the least expensive.

[0178] To keep the reduction system in an air atmosphere, for example,it is desirable that the substances to be processed are piled up inheaps in a yard equipped with drainage.

[0179] The volume of each heap is not specifically determined and mayvary from a few m³ to tens of thousands of m³.

[0180] Each heap is preferably not high. This is because too high heapshave a reduced surface area, and water sprayed thereover or rain fallingthere would find it difficult to spread entirely all through the heaps,resulting in that the reduction of chromium oxides in those heaps takesa long time.

[0181] In the embodiment of the invention mentioned hereinabove, wheresteam is applied to a mixture comprising chromium oxide-containingsubstances, substances that contain sulfur and/or compounds of sulfurhaving a valence less than 6, and slag that contains sulfur and/orcompounds of sulfur having a valence less than 6, preferably employed isany mode of <1> spraying steam over the heaps of the mixture, <2>introducing steam into the heaps of the mixture, or <3> introducingsteam into the heaps of the mixture from below, or even any others.

[0182] The temperature of the steam to be applied to the mixture is notspecifically defined.

[0183] Where steam is applied to the mixture in air, its temperature isgenerally 100° C. However, where steam is introduced into the heaps ofthe mixture under high pressure, the boiling point of water will behigher than 100° C. as the inside of the heaps is under increasedpressure, resulting in that the temperature of the steam that has beenapplied to those heaps will increase, depending on the increasedpressure. Where steam is introduced under high pressure into the mixturein a closed container, the temperature of the steam applied alsoincreases, depending on the increased pressure, as the boiling point ofwater is higher than 100° C. under increased pressure. For example, thetemperature of steam reaches about 180° C. under 10 atmospheres.

[0184] In those embodiments, the steam to be applied to the reductionsystem is not limited only to water vapor, but may be combined with anyother gasses, such as air, N₂ and the like.

[0185] In the embodiments of the invention mentioned hereinabove, <1>blast furnace slag-released water, <2> slag containing sulfur and/orcompounds of sulfur having a valence less than 6, such as non-aged,gradually-cooled blast furnace slag, etc., <3> elementalsulfur-containing substances, and <4> sulfur compounds with a valenceless than 6 may be combined with divalent iron-containing substanceshaving the ability to reduce chromium oxides such as those mentionedhereinabove, and any other reducing agents such as activated charcoaland the like.

[0186] The invention is applicable not only to stainless steel slag,chromium slag, molten slag of industrial waste, sewage sludge, moltenslag of sewage sludge and the like slag, but also to any othersubstances that may release Cr⁶⁺, thereby preventing the release of Cr⁶⁺from those substances.

[0187] Chromium oxide-containing substances to be processed according tothe invention may be pre-treated with steam.

[0188] When slag that releases Cr⁶⁺ in an amount of 10 mg/liter or morein the metal release test as set forth in the Notification No. 46 of theEnvironment Agency of Japan, or slag having a low degree of porosity isprocessed according to the method of spraying blast furnaceslag-released water over the slag or according to the method ofimmersing the slag in blast furnace slag-released water, it takes a longtime to safely stabilize the slag.

[0189] On the other hand, when the slag of that type is processedaccording to the method of applying steam to the slag as mixed withgradually-cooled blast furnace slag, it may be stabilized within a shortperiod of time but its volume increases as this is mixed withgradually-cooled blast furnace slag.

[0190] Taking those problems into consideration, the present inventorshave further studied and made various experiments, and, as a result,have found that, when steam is previously applied to Cr⁶⁺-containingslag (chromium oxide-containing substances), Cr⁶⁺ in the slag can berendered easily reducible, and that, when the thus pre-treated, Cr⁶⁺-containing slag is reduced with blast furnace slag-released water, itcan be stabilized within a shorter period of time thereby completely toprevent the release of Cr⁶⁺ from the thus-stabilized slag. In otherwords, we have found that the pre-treatment with steam shortens theprocessing time and simplifies the reduction process itself, and thatthe method comprising the pre-treatment with steam is industriallyadvantageous and economical.

[0191] Specifically, in those preferred embodiments, steam is previouslyapplied to chromium oxide-containing substances, and then the thuspre-treated substances are reduced in the manner mentioned hereinabove.For example, blast furnace slag-released water is sprayed over thepre-treated, chromium oxide-containing substances; or the pre-treated,chromium oxide-containing substances are immersed in blast furnaceslag-released water.

[0192] These embodiments will be described in more detail hereinafter.

[0193] The present inventors have studied and made various experimentsin order to find out an industrial, simple, economical and effectivemethod capable of processing slag that releases Cr⁶⁺ in an amount of 10mg/liter or more in the metal release test as set forth in theNotification No. 46 of the Environment Agency of Japan, or slag having alow degree of porosity, within a short period of time without increasingthe volume of the processed slag, thereby completely to prevent therelease of Cr⁶⁺ from the thus-processed slag.

[0194] As a result, we have found that, when steam at a high temperatureis applied to the slag of that type, Cr⁶⁺ existing in the slag can bemade easily releasable or easily reducible. We have further found that,when blast furnace slag-released water is sprayed over the thuspre-treated slag or when the thus pre-treated slag is immersed in blastfurnace slag-released water, then the slag can be efficiently andeconomically processed within a short period of time without increasingthe volume of the processed slag, whereby Cr⁶⁺ in the slag is reducedand completely stabilized.

[0195] Specifically, according to those preferred embodiments of theinvention, where steam is previously applied to chromiumoxide-containing slag (chromium oxide-containing substances) and thenblast furnace slag-released water is sprayed over the thus pre-steamedslag, or where steam is previously applied to chromium oxide-containingslag (chromium oxide-containing substances) and then the thuspre-steamed slag is immersed in blast furnace slag-released water, Cr⁶⁺in the slag can be reduced within a short period of time withoutincreasing the volume of the processed slag. Accordingly, thoseembodiments of the invention are extremely economical.

[0196]FIG. 1 shows the relationship between the time of steamingstainless steel slag, which is one example of chromium oxide-containingsubstances, and the amount of Cr⁶⁺ released from the thus-steamed slag.

[0197] Briefly, steam at 100° C. was applied to stainless steel slag,from which Cr⁶⁺ is released, for a predetermined period of time, andthereafter the thus-steamed slag was subjected to the metal release testas set forth in the Notification No. 46 of the Environment Agency ofJapan, and the amount of Cr⁶⁺ released from the slag was measured. Thedata obtained are plotted relative to the steaming time in FIG. 1.

[0198] The slag used in the test had a grain size of from 13.2 to 26.5mm, and was not ground prior to the test.

[0199] As in FIG. 1, the amount of Cr⁶⁺ released from the slag wasgreater after the steaming time of 24 hours than that before thesteaming at 100° C.

[0200] Since steam does not have the ability to oxidize chromium, thetest data obtained herein suggest that the steaming at 100° C. makesCr⁶⁺ in the slag easily releasable, or that is, easily reducible.

[0201] This is because the capillary action of steam applied to the slagwill make Cr⁶⁺ existing in the depth of the slag move to and condense inthe surface layer of the slag.

[0202] Where steam at 100° C. was applied to the slag for a period oftime longer than 24 hours, the amount of Cr⁶⁺ released from the slagdecreased. This is because Cr⁶⁺ that had been made easily releasablefrom the slag by the initial steaming was dissolved by the successivesteaming and removed.

[0203] Accordingly, Cr⁶⁺ could be completely removed from the slag bycontinuous steaming for an extremely long period of time, which,however, is not practical.

[0204] On the basis of those findings noted above, the present inventorshave found a preferred embodiment of the invention whereinCr⁶⁺-containing slag is previously steamed thereby to make Cr⁶⁺ in theslag easily releasable or easily reducible, and thereafter thethus-steamed slag is then reduced with blast furnace slag-releasedwater, for example, water that has been sprayed over non-aged,gradually-cooled blast furnace slag.

[0205] The steam to be used in this embodiment is preferablyhigh-temperature steam, but its temperature is not specifically defined.

[0206] This is because, when steam is applied to Cr⁶⁺-containing slag orthe like in air, its temperature is generally 100° C., but when steam isintroduced under high pressure into a closed container containingtherein Cr⁶⁺-containing slag or the like, the boiling point of waterwill be higher than 100° C. as the inside of the container is underincreased pressure, resulting in that the temperature of the steamhaving been introduced into the container increases, depending on theincreased pressure.

[0207] In this embodiment, the steam to be applied to the reductionsystem is not limited to water vapor only, but may be combined with anyother gases, such as air, N₂ and the like.

[0208] The steaming time is not also specifically defined.

[0209] Depending on the grain size and the porosity of the slag to bepre-steamed, the steaming time within which Cr⁶⁺ existing in the slag ismade releasable or reducible may be determined. In most cases, thesteaming time may fall between 1 and 120 hours.

[0210] The blast furnace slag for use in this embodiment of theinvention is preferably non-aged, gradually-cooled blast furnace slagthat colors water in the color identification test defined in JIS A5015,Appendix 1. As the blast furnace slag-released water for use therein, ingeneral, preferred is water that has been sprayed over non-aged,gradually-cooled blast furnace slag that has been spontaneously aged forless than 3 months.

[0211] For example, preferably used is water that has been sprayed overhot slag that has just been discharged from blast furnaces.

[0212] The effectiveness of the blast furnace slag-released water toreduce Cr⁶⁺ into Cr³+is due to the oxidation of the reducing sulfur ofdifferent types (S²⁻, S⁰, S₂O₃ ²⁻) that exists in the water.

[0213] In this embodiment of the invention, the blast furnaceslag-released water to be used may be combined with any other reducingagents having the ability to reduce Cr⁶⁺.

[0214] For example, ferrous sulfate may be dissolved in the blastfurnace slag-released water to be used therein.

[0215] In this embodiment, the amount of the blast furnace slag-releasedwater to be sprayed over the slag to be reduced, or the time for whichthe slag to be reduced is immersed in the blast furnace slag-releasedwater, and the ratio of the slag to be reduced to the blast furnaceslag-released water in which the slag is immersed are not specificallydefined.

[0216] The preferred values of those parameters may be suitablydetermined, depending on the grain size, the porosity and the Cr⁶⁺content of the slag to be reduced.

[0217] The present invention is especially effectively applied to slagthat may release a large amount, for example, 10 mg/liter or more ofCr⁶⁺, which, however, is not limitative. Needlesstosay, the invention isalso effectively applicable to any and every slag that may release asmall amount of Cr⁶ ⁺.

[0218] The invention is applicable not only to Cr⁶ ⁺-containingstainless steel slag, chromium slag, industrial wastes, slag of moltenwastes and the like, but also to any other substances that may releaseCr⁶ ⁺, thereby preventing the release of Cr⁶⁺ from those substances.

[0219] As its second aspect, the present invention provides a method ofsafely recycling chromium oxide-containing substances such as chromiumoxide-containing slag.

[0220] The present inventors have assiduously studied the problems inthe prior art of recycling chromium oxide-containing substances notedhereinabove, and, as a result, have found that, when chromiumoxide-containing substances are, after having been reduced, mixed with apredetermined amount of sulfur-containing slag, the release of Cr⁶⁺ fromthe resulting mixture is completely prevented, and the mixture can beeffectively used in roadbeds, fillers for use in civil engineeringreclamation works, temporary works, and the like.

[0221] The second aspect of the invention will be described in detailhereinafter.

[0222] The method of reducing chromium oxide-containing substances foruse in the second aspect of the invention is not specifically defined,provided that it is to reduce Cr⁶⁺ existing in chromium oxide-containingsubstances, but is preferably such that the amount of Cr⁶⁺ released fromthe reduced chromium oxide-containing substances, when measuredaccording to the metal release test as set forth in the Notification No.46 of the Environment Agency of Japan, may be not greater than 0.05mg/liter.

[0223] As the reducing method, any of the conventional reducing methodsmentioned hereinabove may be employed, but more preferred is the methodof the first aspect of the invention for reducing chromium oxides notedabove. In the second aspect of the invention, chromium oxide-containingsubstances are, after having been reduced according to the method notedabove, mixed with sulfur-containing slag. In the resulting mixture, therelease of Cr⁶⁺ from the chromium oxide-containing substances iscompletely prevented, and the mixture is effectively useable inroadbeds, fillers for use in civil engineering reclamation works,temporary works, civil engineering and construction materials, and thelike.

[0224] As the sulfur-containing slag for use in the second aspect of theinvention, preferably used is gradually-cooled and aged blast furnaceslag, slag as discharged in the pre-treatment of molten pig iron, theirmixture, and the like.

[0225] The amount of the sulfur-containing slag to be added to thereduced chromium oxide-containing substances is preferably from 0.1 to90 parts by weight relative to 100 parts by weight of the reducedchromium oxide-containing substances.

[0226] When the amount of the sulfur-containing slag added is less than0.1 part by weight, the complete prevention of the release of Cr⁶⁺ fromthe chromium oxide-containing substances will be difficult if thereduction of the chromium oxide-containing substances is incomplete. Onthe other hand, when the amount of the sulfur-containing slag added isgreater than 90 parts by weight, the energy necessary for mixing themwill be larger as the amount of the sulfur-containing slag to be handledis larger. Therefore, the addition of such a large amount ofsulfur-containing slag is unfavorable from the economical viewpoint.

[0227] Regarding the sulfur content of the sulfur-containing slag to beused in the invention, the overall amount of sulfur in the slag, whichis the total of elemental sulfur and all sulfur compounds therein, ispreferably not less than 0.1% by weight; more preferably, the total ofelemental sulfur and compounds of sulfur having a valence less than 6 inthe slag is not less than 0.2% by weight.

[0228] The reason why adding sulfur-containing slag to reduced chromiumoxide-containing substances ensures the complete prevention of therelease of Cr⁶⁺ from the chromium oxide-containing substances is becausethe sulfur component existing in the sulfur-containing slag willdissolve in water thereby acting to reduce Cr⁶ ⁺.

[0229] Specifically, it is believed that S⁰, S₂O₃ ²⁻ and SO₄ ²⁻ in thesulfur-containing slag, such as gradually-cooled and aged blast furnaceslag, slag as discharged in the pre-treatment of molten pig iron or thelike, dissolve in water, whereupon S⁰ and S₂O₃ ²⁻ are oxidized to SO₄ ²⁻while reducing Cr⁶ ⁺.

[0230] In the invention, the sulfur-containing slag is contacted withthe chromium oxide-containing substances in the presence of water,whereupon the slag reduces chromium to a stable chromium compound suchas Cr(OH)₃ or the like, resulting in that the release of chromium oxidesfrom the chromium oxide-containing substances is completely prevented.

[0231] In the invention, the sulfur-containing slag to be added may becombined with any other reducing agents having the ability to reducechromium compounds, such as iron sulfate, activated charcoal and thelike.

[0232] The invention will be described more concretely with reference tothe following Examples, which, however, are not intended to restrict thescope of the invention.

[0233] In the following Examples, the amount of Cr⁶⁺ released fromsamples was measured according to the metal release test as described inthe Notification No. 46 of the Environment Agency of Japan.

EXAMPLE A1

[0234] As chromium oxide-containing substances, herein used werestainless steel slag, chromium slag as discharged during the productionof sodium bichromate, and sewage sludge.

[0235] The stainless steel slag is produced during reduction in thecourse of refining stainless steel.

[0236] As sulfur-containing substances, used herein were non-aged,gradually-cooled blast furnace slag, gradually-cooled blast furnace slagspontaneously aged for 6 months, and blast furnace slag-released water.

[0237] The non-aged, gradually-cooled blast furnace slag is one produced1 week after cooling and grinding blast furnace slag.

[0238] The blast furnace slag-released water is water that has beensprayed over hot blast furnace slag.

[0239] The reducing sulfur concentration in the blast furnaceslag-released water used herein was 0.05% by weight.

[0240] Table 4 shows the chemical compositions of the samples ofnon-aged, gradually-cooled blast furnace slag, stainless steel slag,chromium slag and sewage sludge tested herein, and the amount of Cr⁶⁺released from each sample.

[0241] In Table 4, the amount of Cr⁶⁺ released from each sample ofstainless steel slag, chromium slag and sewage sludge was 1.0 mg/liter,9.7 mg/liter and 0.80 mg/liter, respectively.

[0242] Those chromium oxide-containing substances (samples to bereduced) were processed under the different conditions shown in Table 5.

[0243] The amount of the non-aged, gradually-cooled blast furnace slagadded, which is in Table 5, is in terms of % by weight relative to 100%by weight of each sample to be reduced with it.

[0244] In Table 5, there are shown the experimental data obtained alongwith the conditions employed in the experiments that were conducted.

[0245] The sample to be reduced and the non-aged, gradually-cooled blastfurnace slag were mixed uniformly.

[0246] Each heap of samples was kept in an air atmosphere and weighedabout 100 tons and had a height of 2 m.

[0247] As seen in Table 5, the amount of Cr⁶⁺ released from comparativesamples A1 to A4 which had been kept in an air atmosphere or had beensteamed at 100° C., without adding non-aged, gradually-cooled blastfurnace slag thereto, was not lower than the environmentally acceptablestandard value of 0.05 mg/liter. This is because no reducing agent wasadded to those comparative samples.

[0248] The amount of Cr⁶⁺ released from comparative sample A5, to whichhad been added gradually-cooled blast furnace slag and spontaneouslyaged for 6 months and which had been kept in an air atmosphere, was alsono lower than 0.05 mg/liter.

[0249] As to comparative sample A6, stainless steel slag, which was keptin an air atmosphere while spraying thereover city water of the sameweight as the slag, there was found no significant difference in theamount of Cr⁶⁺ released from it before and after the treatment.

[0250] In contrast to those comparative samples, samples A1 to A16 ofthe invention were processed as follows: [1] Each sample was mixed withnon-aged, gradually-cooled blast furnace slag, and then kept in an airatmosphere (samples A1to A4, A10 and A14 of the invention). [2] Eachsample was mixed with non-aged, gradually-cooled blast furnace slag, andthen kept in an air atmosphere while either one or both of blast furnaceslag-released water and ordinary water was sprayed thereover (sample A7,A8, A9, A12 and A16 of the invention). [3] Each sample was immersed inblast furnace-released water (samples A6, A11 and A15 of the invention).[4] Each sample was mixed with non-aged, gradually-cooled blast furnaceslag, and high-temperature steam was applied thereto (samples A5 and A13of the invention). As a result, the amount of Cr⁶⁺ released from eachsample of the invention thus processed was lower than the environmentalstandard value of 0.05 mg/liter.

[0251] As for sample A17, concerning stainless steel slag of theinvention, over which was sprayed blast furnace slag-released water ofthe same weight as that of the slag, the amount of Cr⁶⁺ released fromthe thus-processed slag was lower than the environmentally acceptablestandard value.

[0252] Although the samples, stainless steel slag, chromium slag andsewage sludge, which had been reduced as described herein according tothe invention, were kept in an air atmosphere for 1 year, they releasedno Cr⁶ ⁺. TABLE 4 Cr⁶⁺ Al₂O₃ Total Fe Total Cr Released CaO (wt. %) SiO₂(wt. %) (wt. %) MgO (wt. %) Total S (wt. %) (wt. %) (wt. %) Water(wt. %)(mg/liter)¹⁾ Non-aged, 42.4 34.2 13.8 7.2 1.09 0.53 — — — gradually-cooled blast furnace slag Stainless 55.6 28.7 4.83 5.0 0.03 2.1 0.6 —1.0 steel slag Chromium 26.8 25.7 15.2 9.8 0.82 7.0 5.2 — 9.7 slagSewage 1.2 13.3 4.9 0.5 0.6 1.2 0.3 75 0.8 sludge

[0253] TABLE 5-1 Non-aged, Gradually- cooled Blast Cr⁶⁺ Released SampleProcessed Furnace Slag (mg/liter)¹⁾ Grain Amount Grain Before After SizeAdded Size Treat- Treat- Type (mm) (wt. %) (mm) Processing Method A²⁾ment ment Comparative stainless ˜25 0 — Kept in air atmosphere for 3months. 0 1.0 0.98 Sample A1 steel slag Comparative chromium ˜25 0 —Kept in air atmosphere for 3 months. 0 9.7 9.4  Sample A2 slagComparative sewage ˜5 0 — Kept in air atmosphere for 3 months. 0 0.80.79 Sample A3 sludge Comparative stainless ˜25 0 — Steamed at 100° C.for 24 hours. 0 1.0 0.60 Sample A4 steel slag Comparative stainless ˜250 — 50 wt. % of gradually-cooled blast furnace slag as 5.0 1.0 0.52Sample A5 steel slag spontaneously aged for 6 months (grain size: ˜25mm) was added, and kept in air atmosphere for 3 months. Comparativestainless ˜25 0 — Sprayed with city water of the same weight as that ofthe 0 1.0 0.97 Sample A6 steel slag sample, for 3 months.³⁾ Sample A1 ofstainless ˜40 50 ˜40 Kept in air atmosphere for 3 months. 5.0 1.0<0.05   the Invention steel slag Sample A2 of stainless ˜25 50 ˜25 Keptin air atmosphere for 3 months. 5.0 1.0 <0.05   the Invention steel slagSample A3 of stainless ˜25 25 ˜25 Kept in air atmosphere for 3 months.2.5 1.0 <0.05   the Invention steel slag Sample A4 of stainless ˜25 50˜25 Kept in air atmosphere for 1 month. 5.0 1.0 <0.05   the Inventionsteel slag Sample A5 of stainless ˜25 5 ˜25 Steamed at 100° C. for 24hours. 0.5 1.0 <0.05   the Invention steel slag Sample A6 of stainless˜25 0 — Immersed in blast furnace slag-released water 0 1.0 <0.05   theInvention steel slag of the same weight as that of the sample for 1week. Sample A7 of stainless ˜25 5 ˜25 Kept in air atmosphere for 1month, while being 0.6 1.0 <0.05   the Invention steel slag sprayed withblast furnace slag-released water of 0.1 times the weight of thesample.⁴⁾

[0254] TABLE 5-2 Non-aged Gradually- cooled Blast Cr⁶⁺ Released SampleProcessed Furnace Slag (mg/liter)¹⁾ Grain Amount Grain Before After SizeAdded Size Treat- Treat- Type (mm) (wt. %) (mm) Processing Method A²⁾ment ment Sample A8 of stainless ˜25 10 ˜25 Kept in air atmosphere for 3months, while 1.0 1.0 <0.05 the Invention steel slag being sprayed withwater of the same weight as that of the sample.³⁾ Sample A9 of stainless˜25 7 ˜25 Kept in air atmosphere for 3 months, while 0.8 1.0 <0.05 theInvention steel slag being sprayed with blast furnace slag- releasedwater of 0.1 times the weight of the sample⁴⁾ and with water of 0.5times the weight of the sample.³⁾ Sample A10 chromium ˜25 300 ˜25 Keptin air atmosphere for 3 months. 3.1 9.7 <0.05 of the slag InventionSample A11 chromium ˜25 0 — Immersed in blast furnace slag-released  09.7 <0.05 of the slag water of 2 times the weight of the sample, forInvention 1 week. Sample A12 chromium ˜25 100 ˜25 Kept in air atmospherefor 1 month, while 1.1 9.7 <0.05 of the slag being sprayed with blastfurnace slag- Invention released water of the same weight as that of thesample.⁴⁾ Sample A13 chromium ˜25 20 ˜25 Steamed at 100° C. for 72hours. 0.2 9.7 <0.05 of the slag Invention Sample A14 sewage ˜5 50 ˜25Kept in air atmosphere for 3 months. 6.3 0.8 <0.05 of the sludgeInvention Sample A15 sewage ˜5 0 — Immersed in blast furnaceslag-released  0 0.8 <0.05 of the sludge water of the same weight asthat of the Invention sample, for 1 week. Sample A16 sewage ˜5 5 ˜25Kept in air atmosphere for 3 months, while 0.8 0.8 <0.05 of the sludgebeing sprayed with blast furnace slag- Invention released water of 0.1times the weight of the slag.⁴⁾ Sample A17 stainless ˜25 0 — Sprayedwith blast furnace slag-released 0.1 1.0 <0.05 of the steel slag waterof 1.0 times the weight of the sample Invention for 3 months.³⁾

EXAMPLE A2

[0255] As chromium oxide-containing substances, there were used hereinstainless steel slag, chromium slag discharged during the production ofsodium bichromate, slag of molten sewage sludge, and stainless steelslag-adhered refractory waste.

[0256] Table 6 shows the chemical compositions of the samples ofchromium oxide-containing substances tested herein, the porosity of eachsample, and the amount of Cr⁶⁺ released from each sample.

[0257] As in Table 6, the amount of Cr⁶⁺ released from each sample ofstainless steel slag A, stainless steel slag B, stainless steel slag C,chromium slag, slag of molten sewage sludge, and stainless steelslag-adhered refractory waste was 6.50 mg/liter, 32.7 mg/liter, 13.2mg/liter, 25.3 mg/liter, 0.8 mg/liter and 0.12 mg/liter, respectively.

[0258] The porosity of each sample of stainless steel slag A, stainlesssteel slag B, stainless steel slag C, chromium slag, and slag of moltensewage sludge was 18%, 25%, 4%, 10% and 8%, respectively.

[0259] Stainless steel slag is generally discharged from a converter,after having been reduced. However, the samples of stainless steel slagused herein were not reduced prior to being discharged.

[0260] As sulfur-containing substances, used herein were non-aged,gradually-cooled blast furnace slag, elemental sulfur, and sinter.

[0261] The non-aged, gradually-cooled blast furnace slag was sampledwithin 1 week after having been cooled and ground.

[0262] Those slag samples to be reduced were steamed under variousconditions shown in Table 7.

[0263] In Table 7 are shown the experimental data obtained along withthe conditions employed.

[0264] As for comparative sample A8, stainless steel slag A, in Table 7,to which was added 10% by weight of non-aged, gradually-cooled blastfurnace slag and which was steamed for 2 days, the amount of Cr⁶⁺released from the processed slag A was reduced, but still was not lessthan 0.05 mg/liter.

[0265] As for comparative sample A9, stainless steel slag B, to whichwas added 10% by weight of non-aged, gradually-cooled blast furnace slagand which was steamed for 3 days, the amount of Cr⁶⁺ released from theprocessed slag B was reduced, but still was not less than 0.05 mg/liter.

[0266] As opposed to those comparative samples, the amount of Cr⁶⁺released from samples A18 to A26 of the invention, to which had beenadded a small amount of a sulfur-containing substance having a highsulfur content and which had been steamed for 3 days or shorter, wasless than 0.05 mg/liter.

[0267] As for comparative sample A10, stainless steel slag C, to whichwas added 20% by weight of non-aged, gradually-cooled blast furnace slagand which was steamed for 3 days, the amount of Cr⁶⁺ released from theprocessed slag C was reduced, but still was not less than 0.05 mg/liter.As opposed to this comparative sample, the amount of Cr⁶⁺ released fromsample A25 of the invention, to which had been added 5% by weight ofnon-aged, gradually-cooled blast furnace slag and 1% by weight of sulfurand which had been steamed for 3 days, was less than 0.05 mg/liter.TABLE 6 Cr⁶⁺ Total Fe Total Cr Released CaO (wt. %) SiO₂ (wt. %) Al₂O₃(wt. %) MgO (wt. %) Total S (wt. %) (wt. %) (wt. %) Porosity (%)(mg/liter)¹⁾ Stainless 57.0 20.4 3.8 4.9 0.03 3.6 4.5 18 6.50 steel slagA Stainless 61.3 17.5 2.8 2.6 0.03 4.3 6.2 25 32.7 steel slag BStainless 58.3 18.8 3.3 2.5 0.03 4.2 5.5  4 13.2 steel slag C Chromium28.5 24.9 13.8 9.5 0.91 6.5 7.4 10 25.3 slag Slag of 10.5 47.5 20.2 3.02.5 5.2 1.6  8 0.80 molten sewage sludge Stainless 9.5 13.2 24.4 37.10.08 4.5 0.3 — 0.12 steel slag- adhered refractory waste

[0268] TABLE 7-1 Non-aged, Gradually- Cr⁶⁺ Released cooled (mg/liter)¹⁾Sample Processed Blast Furnace Slag After Grain Size Amount Grain SizeBefore Treat- Type (mm) Added (wt. %) (mm) Processing Method A²⁾Treatment ment Comparative stainless ˜40  0 — Steamed at 100° C. for 3days.   0 6.5 5.20 Sample 7 steel slag A Comparative stainless ˜40 10˜25 Steamed at 100° C. for 2 days. 0.15 6.5 0.25 Sample 8 steel slag AComparative stainless ˜25 10 ˜25 Steamed at 100° C. for 3 days. 0.0332.7 1.31 Sample 9 steel slag B Comparative stainless ˜40 20 ˜25 Steamedat 100° C. for 3 days. 0.15 13.2 1.52 Sample 10 steel slag C Comparativechromium ˜25 10 ˜25 Steamed at 100° C. for 3 days. 0.04 25.3 1.95 Sample11 slag

[0269] TABLE 7-2 Non-aged, Gradually-cooled Blast Furnace Cr⁶⁺ ReleasedSample Processed Slag Additive (mg/liter)¹⁾ Grain Amount Grain AmountBefore After Size Added Size Added Treat- Treat- Type (mm) (wt. %) (mm)Type (wt. %) Processing Method A²⁾ ment ment Sample A18 of stainlesssteel ˜40 0 — sulfur 0.5 Steamed at 100° C. for 2 days. 0.38 6.50 <0.05the Invention slag A Sample A19 of stainless steel ˜40 0 — sinter⁵⁾ 2Steamed at 100° C. for 2 days. 1.54 6.50 <0.05 the Invention slag ASample A20 of stainless steel ˜40 0 — sulfur 2 Steamed at 100° C. for 3days. 0.31 32.7 <0.05 the Invention slag B Sample A21 of chromium slag˜25 0 — sulfur 2 Steamed at 100° C. for 3 days. 0.40 25.3 <0.05 theInvention Sample A22 of slag of molten ˜25 0 — sulfur 0.5 Steamed at100° C. for 2 days. 3.13 0.80 <0.05 the Invention sewage sludge SampleA23 of stainless steel ˜40 0 — sulfur 0.5 Steamed at 100° C. for 1 day.20.8 0.12 <0.05 the Invention slag-adhered refractory waste Sample A24of stainless steel ˜25 5 ˜25 sulfur 1 Steamed at 100° C. for 3 days.0.17 32.7 <0.05 the Invention slag B Sample A25 of stainless steel ˜40 5˜25 sulfur 1 Steamed at 100° C. for 3 days. 0.42 13.2 <0.05 theInvention slag C Sample A26 of chromium slag ˜25 5 ˜25 sulfur 1 Steamedat 100° C. for 3 days. 0.22 25.3 <0.05 the Invention

EXAMPLE A3

[0270] The same chromium oxide-containing substances as in Example A2were processed under different immersion conditions shown in Table 8.

[0271] As sulfur-containing substances, there were used herein blastfurnace slag-released water, elementary sulfur, sinter, hydrogensulfide, and sodium thiosulfate; and as a divalent iron-containingcompound, there was used ferrous sulfate.

[0272] In Table 8, there are shown the experimental data obtained alongwith the conditions employed.

[0273] As for comparative samples A12 to A15 in Table 8, which wereimmersed in blast furnace slag-released water having a reducing sulfurconcentration of 0.03% by weight for 3 to 5 days, the amount of Cr⁶⁺released from the processed samples was reduced, but still was not lessthan 0.05 mg/liter.

[0274] In contrast to those comparative samples, samples A27 to A31 ofthe invention were immersed in blast furnace slag-released water, towhich or over which had been added or sprayed any of elemental sulfur,sinter, sodium thiosulfate and hydrogen sulfate (that is, any ofelemental sulfur-containing substances or compounds of sulfur having avalence less than 6) thereby to control the reducing sulfur content ofthe resulting water, for 5 days or less. As a result, the amount of Cr⁶⁺released from those processed samples of the invention was less than0.05 mg/liter.

[0275] As for sample A32 of the invention, it was immersed in blastfurnace slag-released water, to which had been added ferrous sulfate,for 3 days. The amount of Cr⁶⁺ released from this sample A32 was alsoless than 0.05 mg/liter. TABLE 8-1 Non-aged, Gradually- cooled BlastCr⁶⁺ Released Furnace Slag (mg/liter)¹⁾ Sample Processed Amount BeforeAfter Grain Size Added Grain Treat- Treat- Type (mm) (wt. %) Size (mm)Processing Method A²⁾ ment ment Comparative stainless steel ˜40 0 —Immersed in blast furnace slag-released water⁴⁾ of 0 6.50 1.76 SampleA12 slag A 0.5 times the weight of the sample, for 3 days. Comparativestainless steel ˜25 0 — Immersed in blast furnace slag-released water⁴⁾of 0 32.7 6.13 Sample A13 slag B 3.0 times the weight of the sample, for3 days. Comparative slag of molten ˜25 0 — Immersed in blast furnaceslag-released water⁴⁾ of 0 0.80 0.35 Sample A14 sewage sludge 0.5 timesthe weight of the sample, for 5 days. Comparative stainless steel ˜40 0— Immersed in blast furnace slag-released water⁴⁾ of 0 0.12 0.06 SampleA15 slag-adhered 0.5 times the weight of the sample, for 3 days.refractory waste

[0276] TABLE 8-2 Non-aged, Gradually- cooled Blast Cr⁶⁺ Released FurnaceSlag (mg/liter)¹⁾ Sample Processed Amount Grain Before After Grain SizeAdded Size Treat- Treat- Type (mm) (wt. %) (mm) Processing Method A²⁾ment ment Sample A27 of stainless ˜40 0 — Immersed in blast furnaceslag-released water⁶⁾ of 0.5 0 6.50 <0.05 the Invention steel slag Atimes the weight of the sample, to which had been added sinter⁷⁾ to makethe water have a reducing sulfur concentration of 1 g/liter, for 3 days.Sample A28 of stainless ˜40 0 — Immersed in blast furnace slag-releasedwater⁶⁾ of 0.5 0 6.50 <0.05 the Invention steel slag A times the weightof the sample, over which had been sprayed hydrogen sulfide to make thewater have a reducing sulfur concentration of 2 g/liter, for 3 days.Sample A29 of stainless ˜25 0 — Immersed in blast furnace slag-releasedwater⁶⁾ of 0.5 0 32.7 <0.05 the Invention steel slag B times the weightof the sample, to which had been added sodium thiosulfate to make thewater have a reducing sulfur concentration of 10 g/liter, for 5 days.Sample A30 of slag of ˜25 0 — Immersed in blast furnace slag-releasedwater⁶⁾ of 0.5 0 0.80 <0.05 the Invention molten times the weight of thesample, to which had been sewage added sulfur to make the water have areducing sulfur sludge concentration of 0.4 g/liter, for 3 days. SampleA31 of stainless ˜40 0 — Immersed in blast furnace slag-released water⁶⁾of 0.4 0 0.12 <0.05 the Invention steel slag- times the weight of thesample, to which had been adhered added sulfur to make the water have areducing sulfur refractory concentration of 0.4 g/liter, for 3 days.waste Sample A32 of stainless ˜40 0 — Immersed in blast furnaceslag-released water⁶⁾ of 0.5 0 6.50 <0.05 the Invention steel slag Atimes the weight of the sample, to which had been added ferrous sulfate,for 3 days.

Example B

[0277] As chromium oxide-containing substances which are to be reducedherein, there were used stainless steel slag, chromium slag asdischarged during the production of sodium bichromate, and slag ofmolten sewage sludge.

[0278] The stainless steel slag is one produced in a stainless steelrefining process operated under such conditions that the slag dischargedcontains releasable Cr⁶ ⁺.

[0279] Table 9 shows the chemical compositions of the samples ofstainless steel slag, chromium slag and slag of molten sewage sludgetested herein, the porosity of each sample, and the amount of Cr⁶⁺released from each sample.

[0280] As in Table 9, the amount of Cr⁶⁺ released from each sample ofstainless steel slag, chromium slag, slag A of molten sewage sludge(porosity: 8%), and slag B of molten sewage sludge (porosity: 2%) was10.5 mg/liter, 25.3 mg/liter, 0.8 mg/liter and 0.27 mg/liter,respectively.

[0281] Those slag samples to be reduced were processed under the variousconditions shown in Table 10. The amount of Cr⁶⁺ released from each ofthe thus-processed sample was measured.

[0282] The data obtained are shown in Table 10.

[0283] The amount of each slag sample processed was 100 tons/batch.Blast furnace slag-released water was used herein, which is water thathas been sprayed over hot, non-aged blast furnace slag. This had a totalsulfur concentration of 0.50% by weight.

[0284] Steam was introduced into each sample from below, and itstemperature was 100° C.

[0285] As in Table 10, even though non-aged blast furnace slag-releasedwater was sprayed over comparative sample B1, stainless steel slag whichreleased 10.5 mg/liter of Cr⁶⁺ prior to the treatment, the amount ofCr⁶⁺ released from the thus-processed sample B1 was still not less thanthe environmentally acceptable standard value of 0.05 mg/liter.

[0286] As for comparative samples B2 to B5, stainless steel slag,chromium slag and slag of molten sewage sludge, even though these wereimmersed in non-aged blast furnace slag-released water for 7 days, theamount of Cr⁶⁺ released from the thus-processed samples was still notless than 0.05 mg/liter.

[0287] In contrast to those comparative samples, samples B1 to B9 of theinvention were processed by steaming them followed by spraying thereovernon-aged blast furnace slag-released water, or by steaming them followedby immersing them in non-aged blast furnace slag-released water. As aresult, the amount of Cr⁶⁺ released from the thus-processed samples ofthe invention was less than the environmentally acceptable standardvalue of 0.05 mg/liter.

[0288] Although samples B1 to B9, stainless steel slag, chromium slagand slag of molten sewage sludge, which had been reduced hereinaccording to the invention, were kept in air for 1 year, they releasedno Cr⁶ ⁺. TABLE 9 Cr⁶⁺ Total Fe Total Cr Porosity Released CaO (wt. %)SiO₂ (wt. %) Al₂O₃ (wt. %) MgO (wt. %) Total S (wt. %) (wt. %) (wt. %)(%) (mg/liter)(*) Stainless 59.2 17.3  4.2 3.5 0.03 4.1 5.7 14 10.5Steel Slag Chromium 28.5 24.9 13.8 9.5 0.91 6.5 7.4 10 25.3 Slag Slag Aof 10.5 47.5 20.2 3.0 2.5 5.2 1.6  8 0.80 Molten Sewage Sludge Slag B of10.5 47.5 20.2 3.0 2.5 5.2 1.6  2 0.27 Molten Sewage Sludge

[0289] TABLE 10 Cr⁶⁺ Released (mg/liter)¹⁾ Sample Processed Before AfterGrain Size Treat- Treat- Type (mm) Processing Method ment mentComparative stainless steel slag ˜25 Sprayed with blast furnaceslag-released water of 0.1 times the weight of the 10.5 2.21 sample B1sample, at intervals of 5 days for a total of 10 times. Comparativestainless steel slag ˜25 Immersed in blast furnace slag-released waterof 5 times the weight of the sample, 10.5 0.80 sample B2 for 7 days.Comparative chromium slag ˜25 Immersed in blast furnace slag-releasedwater of 5 times the weight of the sample, 25.3 4.36 sample B3 for 7days. Comparative slag A of molten ˜5 Immersed in blast furnaceslag-released water of the same weight as that of the 0.80 0.37 sampleB4 sewage sludge sample, for 7 days. Comparative slag B of molten ˜5Immersed in blast furnace slag-released water of the same weight as thatof the 0.27 0.10 sample B5 sewage sludge sample, for 7 days. Comparativestainless steel slag ˜25 Steamed at 100° C. for 24 hours. 10.5 14.2sample B6 Comparative chromium slag ˜25 Steamed at 100° C. for 24 hours.25.3 32.4 sample B7 Comparative slag A of molten ˜5 Steamed at 100° C.for 24 hours. 0.80 0.98 sample B8 sewage sludge Comparative slag B ofmolten ˜5 Steamed at 100° C. for 24 hours. 0.27 0.98 sample B9 sewagesludge Sample B1 of stainless steel slag ˜25 Steamed at 100° C. for 24hours, and then sprayed once with blast furnace slag- 10.5 <0.05   theInvention released water of 0.1 times the weight of the sample. SampleB2 of stainless steel slag ˜25 Steamed at 100° C. for 24 hours, and thenimmersed in blast furnace slag-released 10.5 <0.05   the Invention waterof 0.5 times the weight of the sample for 1 day. Sample B3 of stainlesssteel slag ˜40 Steamed at 100° C. for 72 hours, and then immersed inblast furnace slag-released 10.5 <0.05   the Invention water of 0.5times the weight of the sample for 5 days. Sample B4 of chromium slag˜25 Steamed at 100° C. for 24 hours, and then sprayed with blast furnaceslag-released 25.3 <0.05   the Invention water of 0.1 times the weightof the sample at intervals of 5 days for a total of 3 times. Sample B6of slag A of molten ˜5 Steamed at 100° C. for 72 hours, and thenimmersed in blast furnace slag-released 0.80 <0.05   the Inventionsewage sludge water of 0.5 times the weight of the sample for 5 daysSample B7 of slag A of molten ˜5 Steamed at 100° C. for 24 hours, andthen sprayed once with blast furnace slag- 0.80 <0.05   the Inventionsewage sludge released water of 0.1 times the weight of the sample.Sample B8 of slag B of molten ˜5 Steamed at 100° C. for 72 hours, andthen immersed in blast furnace slag-released 0.27 <0.05   the Inventionsewage sludge water of 0.5 times the weight of the sample for 5 daysSample B9 of slag B of molten ˜5 Steamed at 100° C. for 24 hours, andthen sprayed once with blast furnace slag- 0.27 <0.05   the Inventionsewage sludge released water of 0.1 times the weight of the sample.

EXAMPLE C

[0290] Comparative Samples C1 and C2:

[0291] As chromium oxide-containing substances, there were used herein(1) stainless steel slag, and (2) stainless steel slag-adheredrefractory.

[0292] The stainless steel slag is one produced during reduction in thecourse of refining stainless steel.

[0293] The amount of Cr⁶⁺ released from these chromium oxide-containingsubstances is as follows:

[0294] (1) Stainless steel slag: 10.5 mg/liter

[0295] (2) Stainless steel slag-adhered refractory: 0.12 mg/liter

[0296] These chromium oxide-containing substances were reduced accordingto the method C or D shown in Table 11.

[0297] In Table 12, there is shown the rejection rate of the reducedsamples, from which the amount of Cr⁶⁺ released was more than 0.05mg/liter, along with the reducing method employed.

[0298] As in Table 12, the rejection rate of the reduced samples ofstainless steel slag and stainless steel slag-adhered refractory, fromwhich the amount of Cr⁶⁺ released was more than 0.05 mg/liter, namely0.10%.

[0299] Comparative Samples C3 to C6:

[0300] As chromium oxide-containing substances, there were used hereinthe same stainless steel slag and stainless steel slag-adheredrefractory as in comparative samples C1 and C2, and chromium slag andslag of molten sewage sludge.

[0301] The amount of Cr⁶⁺ released from the latter two chromiumoxide-containing substances is as follows:

[0302] (1) Chromium slag: 25.3 mg/liter

[0303] (2) Slag of molten sewage sludge: 0.80 mg/liter

[0304] These chromium oxide-containing substances were reduced accordingto the method B, C or D shown in Table 11.

[0305] 0.05 or 0.09 part by weight of aged, gradually-cooled blastfurnace slag, or slag as discharged in the pre-treatment of molten pigiron, was added to 100 parts by weight of each of the thus-reducedchromium oxide-containing substances, and mixed.

[0306] Regarding the sulfur content of the aged, gradually-cooled blastfurnace slag used herein, the total of elementary sulfur and sulfur ofsulfur compounds in the slag was 0.85% by weight, and the total ofelementary sulfur and sulfur having a valence less than 6 therein was0.42% by weight.

[0307] Regarding the sulfur content of the slag as discharged in thepre-treatment of molten pig iron, which was also used herein, the totalof elementary sulfur and sulfur of sulfur compounds in the slag was0.40% by weight, and the total of elementary sulfur and sulfur having avalence less than 6 therein was 0.12% by weight.

[0308] In Table 12, there is shown the rejection rate of the mixturesamples, in which the amount of Cr⁶⁺ released was more than 0.05mg/liter, along with the reducing method employed, and the type and theamount of the sulfur-containing slag added.

[0309] As shown in Table 12, when the sulfur-containing slag was addedto the reduced chromium oxide-containing substance in an amount of lessthan 0.1 parts by weight, relative to 100 parts by weight of the reducedchromium oxide-containing substance, the rejection rate of the mixturesamples, in which the amount of Cr⁶⁺ released was over 0.05 mg/liter,was 0.05%.

[0310] Sample C1 of the Invention:

[0311] The same stainless steel slag as that in comparative sample C1was reduced according to the method C shown in Table 11.

[0312] 0.1 part by weight of aged, gradually-cooled blast furnace slagwas added to 100 parts by weight of the thus-reduced stainless steelslag, and mixed.

[0313] Regarding the sulfur content of the aged, gradually-cooled blastfurnace slag used herein, the total of elementary sulfur and sulfur ofsulfur compounds in the slag was 0.85% by weight, and the total ofelementary sulfur and sulfur having a valence less than 6 therein was0.42% by weight.

[0314] As shown in Table 12, the rejection rate of the mixture samples,from which the amount of Cr⁶⁺ released was more than 0.05 mg/liter, was0.00%.

[0315] Samples C2 to C24 of the Invention:

[0316] As chromium oxide-containing substances, there were used herein(1) stainless steel slag, (2) stainless steel slag-adhered refractory,(3) chromium slag, and (4) slag of molten sewage sludge.

[0317] The amount of Cr6+released from these chromium oxide-containingsubstances is as follows:

[0318] (1) Stainless steel slag: 10.5 mg/liter

[0319] (2) Stainless steel slag-adhered refractory: 0.12 mg/liter

[0320] (3) Chromium slag: 25.3 mg/liter

[0321] (4) Slag of molten sewage sludge: 0.80 mg/liter

[0322] These chromium oxide-containing substances were reduced accordingto the method A, B, C or D shown in Table 11. From 0.1 to 90 parts byweight of aged, gradually-cooled blast furnace slag, or slag asdischarged in the pre-treatment of molten pig iron, was added to 100parts by weight of each of the thus-reduced chromium oxide-containingsubstances, and mixed.

[0323] Regarding the sulfur content of the aged, gradually-cooled blastfurnace slag used herein, the total of elementary sulfur and sulfur ofsulfur compounds in the slag was 0.85% by weight, and the total ofelementary sulfur and sulfur having a valence less than 6 therein was0.42% by weight.

[0324] Regarding the sulfur content of the slag as discharged in thepre-treatment of molten pig iron, which was also used herein, the totalof elementary sulfur and sulfur of sulfur compounds in the slag was0.40% by weight, and the total of elementary sulfur and sulfur having avalence less than 6 therein was 0.12% by weight.

[0325] In Table 12, there is shown the rejection rate of the mixturesamples, from which the amount of Cr⁶⁺ released was more than 0.05mg/liter, along with the reducing method employed, and the type and theamount of the sulfur-containing slag added.

[0326] As shown in Table 12, the rejection rate of the mixture samples,from which the amount of Cr⁶⁺ released was more than 0.05 mg/liter, was0.00%. Comparing the samples of the invention with the comparativesamples, it will be seen that the rejection rate of the former issignificantly lower than that of the latter, namely from 0.05 to 0.10%.The rejection rate as referred to herein is one parameter in recyclingchromium oxide-containing wastes as materials in roadbeds, materials intemporary works, fillers in civil engineering reclamation works and thelike.

[0327] In this Example, non-aged, gradually-cooled blast furnace slag,and blast furnace slag-released water were used for reducing thechromium oxide-containing substances prior to adding thesulfur-containing slag to the reduced substances. In the presentinvention, however, the reduction of chromium oxide-containingsubstances is not specifically defined. TABLE 11 Reduction of ChromiumOxide-containing Substances before Addition thereto of Sulfur-containingSlag A Mixed with non-aged, gradually-cooled blast furnace slag, andthen sprayed with water. B Immersed in blast furnace slag-released waterfor 1 week. C Mixed with non-aged, gradually-cooled blast furnace slag,and then steamed at 100° C. for 24 hours. D Sprayed with blast furnaceslag-released water.

[0328] TABLE 12 Reduction before Mixed with Sulfur-containing Slag AddedRejection Sulfur-containing Amount Added Ratio Sample Processed Slag(*)Type (wt. pts.)(**) (%)(***) Comparative stainless steel slag C none 00.10 Sample C1 Comparative stainless steel slag-adhered refractory Dnone 0 0.10 Sample C2 Comparative stainless steel slag C aged,gradually-cooled blast furnace slag 0.05 0.05 Sample C3 Comparativestainless steel slag-adhered refractory D slag as discharged inpre-treatment of pig 0.09 0.05 Sample C4 iron melt Comparative chromiumslag C aged, gradually-cooled blast furnace slag 0.05 0.05 Sample C5Comparative slag of molten sewage sludge B slag as discharged inpre-treatment of pig 0.09 0.05 Sample C6 iron melt Sample C1 ofstainless steel slag C aged, gradually-cooled blast furnace slag 0.10.00 the Invention Sample C2 of stainless steel slag-adhered refractoryD slag as discharged in pre-treatment of pig 0.1 0.00 the Invention ironmelt Sample C3 of chromium slag C aged, gradually-cooled blast furnaceslag 0.1 0.00 the Invention Sample C4 of slag of molten sewage sludge Bslag as discharged in pre-treatment of pig 0.1 0.00 the Invention ironmelt Sample C5 of stainless steel slag D aged, gradually-cooled blastfurnace slag 5.0 0.00 the Invention Sample C6 of stainless steelslag-adhered refractory D slag as discharged in pre-treatment of pig 5.00.00 the Invention iron melt Sample C7 of chromium slag D aged,gradually-cooled blast furnace slag 5.0 0.00 the Invention Sample C8 ofslag of molten sewage sludge D slag as discharged in pre-treatment ofpig 5.0 0.00 the Invention iron melt Sample C9 of stainless steel slag Baged, gradually-cooled blast furnace slag 0.1 0.00 the Invention SampleC10 of stainless steel slag-adhered refractory B slag as discharged inpre-treatment of pig 0.1 0.00 the Invention iron melt Sample C11 ofchromium slag B aged, gradually-cooled blast furnace slag 0.1 0.00 theInvention Sample C12 of slag of molten sewage sludge A slag asdischarged in pre-treatment of pig 0.1 0.00 the Invention iron meltSample C13 of stainless steel slag B aged, gradually-cooled blastfurnace slag 5.0 0.00 the Invention Sample C14 of stainless steelslag-adhered refractory B slag as discharged in pre-treatment of pig 5.00.00 the Invention iron melt Sample C15 of chromium slag B aged,gradually-cooled blast furnace slag 5.0 0.00 the Invention Sample C16 ofslag of molten sewage sludge B slag as discharged in pre-treatment ofpig 5.0 0.00 the Invention iron melt Sample C17 of stainless steel slagC aged, gradually-cooled blast furnace slag 2.0 0.00 the InventionSample C18 of stainless steel slag-adhered refractory C slag asdischarged in pre-treatment of pig 2.0 0.00 the Invention iron meltSample C19 of chromium slag C aged, gradually-cooled blast furnace slag2.0 0.00 the Invention Sample C20 of slag of molten sewage sludge C slagas discharged in pre-treatment of pig 2.0 0.00 the Invention iron meltSample C21 of stainless steel slag C slag as discharged in pre-treatmentof pig 90 0.00 the Invention iron melt Sample C22 of stainless steelslag-adhered refractory B slag as discharged in pre-treatment of pig 900.00 the Invention iron melt Sample C23 of chromium slag C aged,gradually-cooled blast furnace slag 90 0.00 the Invention Sample C24 ofslag of molten sewage sludge A aged, gradually-cooled blast furnace slag90 0.00 the Invention

[0329] Although the invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed is:
 1. A method for processing chromium oxide-containingsubstances, comprising the steps of: obtaining a source of blast furnaceslag-released water having been used for cooling blast furnace slag, thewater having therein at least one member selected from the groupconsisting of sulfur and compounds of sulfur having a valence less than6; and immersing chromium oxide-containing substances in the obtainedblast furnace slag-released water, wherein the immersion of the chromiumoxide-containing substances in the obtained blast furnace slag-releasedreduces chromium oxides in the chromium oxide-containing substanceswhile avoiding increasing the volume of the chromium oxide-containingsubstances.
 2. The method for processing chromium oxide-containingsubstances as claimed in claim 1, wherein prior to the step of immersingchromium oxide-containing substances in the obtained blast furnaceslag-released water, the method comprises the further step addingsubstances that contain said at least one member to the obtained water.3. The method of claim 1, wherein said immersing step is performed atatmospheric temperature.
 4. A method of utilizing chromiumoxide-containing substances, comprising the steps of: reducing thechromium oxide-containing substances; selecting the reduced chromiumoxide-containing substances of approximately 25 mm in grain size; andobtaining a roadbed mixture satisfying the grain size distribution ofJIS A5015 (Japanese Industrial Standard) by mixing together 0.1 to 90parts by weight of sulfur-containing slag with 100 parts by weight ofthe reduced, chromium oxide-containing substances.